Effects of subsidy quality on reciprocal subsidies: how leaf litter species changes frog biomass export.

Spatial subsidies are resources transferred from one ecosystem to another and which can greatly affect recipient systems. Increased subsidy quantity is known to increase these effects, but subsidy quality is likely also important. We examined the effects of leaf litter quality (varying in nutrient and tannin content) in pond mesocosms on gray treefrog (Hyla versicolor) biomass export, as well as water quality and ecosystem processes. We used litter from three different tree species native to Missouri [white oak (Quercus alba), northern red oak (Quercus rubra), and sugar maple (Acer saccharum)], one non-native tree [white pine (Pinus strobus)], and a common aquatic grass [prairie cordgrass (Spartina pectinata)]. We found that leaf litter species affected almost every variable we measured. Gray treefrog biomass export was greatest in mesocosms with grass litter and lowest with white oak litter. Differences in biomass export were affected by high tannin concentrations (or possibly the correlated variable, dissolved oxygen) via their effects on survival, and by primary production, which altered mean body mass. Effects of litter species could often be traced back to the characteristics of the litter itself: leaf nitrogen, phosphorus, and tannin content, which highlights the importance of plant functional traits in affecting aquatic ecosystems. This work and others stress that changes in forest species composition could greatly influence aquatic systems and aquatic-terrestrial linkages.

Effects of leachate from tree leaves and grass litter on tadpoles.

Tree species composition can change as a result of succession, climate change, fire suppression, and invasive species. These changes clearly affect forests, but they can also affect aquatic ecosystems based on differences in the input quality of leaf litter, such as plant secondary compounds. These compounds vary in type and concentration depending on species and can be toxic to aquatic organisms. To examine toxic effects on Pseudacris maculata and Pseudacris crucifer tadpoles, we conducted 60-d laboratory experiments to compare leaf litter leachate from a dominant canopy species (red oak) and nonnative species (white pine) with an aquatic grass (prairie cordgrass) and plain water control. An additional experiment examined the effects of white pine on Bufo americanus, Hyla versicolor, and tannin concentrations in natural ponds. Compared with the control and grass, tree leaf extracts resulted in reduced tadpole survival. Leached compounds from pine reduced tadpole survival to 3 d or less. Tadpoles were able to metamorphose in significant numbers only from the two controls. The lowered survival with the red oak treatment might have been caused by lowered dissolved oxygen or high tannin concentrations. However, pine is known to have high concentrations of toxic monoterpenes, which should be investigated further. We found that tannin concentrations in natural ponds were much lower than the test concentrations, indicating that these results may represent worst-case scenarios or unrealistic concentrations.